podman executable can be overridden

[manics]

This allows docker/podman compatible CLIs to be used.

Closes #66

[manics]

This isn't as straightforward as I thought

$ repo2docker --engine podman --PodmanEngine.podman_executable=nerdctl https://github.com/binder-examples/requirements
WARNING: AppArmor profile "nerdctl-default" is not loaded.
         Use 'sudo nerdctl apparmor load' if you prefer to use AppArmor with rootless mode.
         This warning is negligible if you do not intend to use AppArmor.
WARNING: No cpu cfs period support
WARNING: No cpu cfs quota support
WARNING: No cpu shares support
WARNING: No cpuset support
WARNING: IPv4 forwarding is disabled
WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled
Picked Git content provider.
Cloning into '/tmp/repo2dockerl8akz7ru'...
remote: Enumerating objects: 11, done.
remote: Counting objects: 100% (11/11), done.
remote: Compressing objects: 100% (7/7), done.
remote: Total 11 (delta 0), reused 9 (delta 0), pack-reused 0
Receiving objects: 100% (11/11), 398.16 KiB | 2.64 MiB/s, done.
WARNING: AppArmor profile "nerdctl-default" is not loaded.
         Use 'sudo nerdctl apparmor load' if you prefer to use AppArmor with rootless mode.
         This warning is negligible if you do not intend to use AppArmor.
WARNING: No cpu cfs period support
WARNING: No cpu cfs quota support
WARNING: No cpu shares support
WARNING: No cpuset support
WARNING: IPv4 forwarding is disabled
WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled
Traceback (most recent call last):
  File "/home/ubuntu/venv/bin/repo2docker", line 8, in <module>
    sys.exit(main())
  File "/home/ubuntu/venv/lib/python3.10/site-packages/repo2docker/__main__.py", line 474, in main
    r2d.start()
  File "/home/ubuntu/venv/lib/python3.10/site-packages/repo2docker/app.py", line 891, in start
    self.build()
  File "/home/ubuntu/venv/lib/python3.10/site-packages/repo2docker/app.py", line 791, in build
    if self.find_image():
  File "/home/ubuntu/venv/lib/python3.10/site-packages/repo2docker/app.py", line 755, in find_image
    for image in client.images():
  File "/home/ubuntu/venv/lib/python3.10/site-packages/repo2podman/podman.py", line 445, in images
    images = json.loads(lines)
  File "/usr/lib/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.10/json/decoder.py", line 340, in decode
    raise JSONDecodeError("Extra data", s, end)
json.decoder.JSONDecodeError: Extra data: line 2 column 1 (char 320)